A variety of approaches to optical imaging and analysis have been used for many different applications. For example, the endoscope is a useful tool for a variety of applications such as biological research, medical diagnostics, and for image guidance in surgical procedures. Conventional endoscopes utilize a white light source to illuminate a sample and reflected light to visualize the same sample. Such conventional endoscopes are typically limited, however, to visualizing the surface of a sample or to surface inspection within a hollow tissue cavity.
Many applications for which optical analysis would be beneficial are subject to a variety of limitations. For example, space constraints in many applications limit the use of certain tools or approaches that are not generally scaleable in a manner that would facilitate use with these applications. In addition, while certain approaches have been useful in applications characterized by small space constraints, these approaches are often limited in their ability to achieve desirable results, or by their ability for use with certain applications such as with the analysis of tissue in live beings.
One type of tissue that has been particularly difficult to access for examination and surgical intervention is cochlear tissue in the inner ear. In particular, visualization of structures inside the inner ear in vivo has been limited by their small size and inaccessible location deep in the temporal bone.
Analysis of cochlear tissue is important; it has been estimated that 50% of adults living in industrialized countries will suffer some permanent hearing loss by 75 years of age. For instance, hearing loss related to inner ear disorders is a widespread problem that impacts about 28 million Americans. Yet, otologists have been severely limited in their ability to diagnose even common pathology, let alone to treat disease in a non-destructive manner For patients with sudden hearing loss or acute vertigo, otologists are usually forced to list numerous potential causes before admitting the cause is unknown. Treatment is thus compromised, since the various potential causes of hearing loss may respond quite differently to specific interventions.
Much hearing loss is thought to stem from damage to the sensory receptor cells in the cochlea. These hair cells are found in a tonotopically organized ribbon 4-5 cells wide and about 5,000 cells long that coils along the length of the cochlea, with higher frequencies at the basal end and lower frequencies at the apical end. Sound vibrates the basilar membrane on which the hair cells ride, and causes a relative shearing between the apical surfaces of the cells and a membrane contacting the tops of their mechanosensory bundles. This mechanical stimulation directly opens ion channels found at the tips of the stereocilia that constitute the bundle.
Previous approaches to assessing damage to the cochlea has been assessed either indirectly in vivo using electrical methods to monitor the auditory nerve firings or the auditory brainstem response (ABR), or more directly in vitro by excising sections of the cochlea and studying the anatomy. However, these approaches have been limited in resolution and, for the latter approach, are not possible for use with treating live beings.
The above and other issues have presented challenges to optical analysis approaches and, in particular, to optical imaging in applications exhibiting relatively small space such as for endoscopic and microscopic applications.